(Not Applicable)
The present invention relates to a portable isocapnia circuit and a method to set and stablize end tidal and arterial PCO2 despite varying levels of minute ventilation.
Venous blood returns to the heart from the muscles and organs partially depleted of oxygen (O2) and a full complement of carbon dioxide (CO2). Blood from various parts of the body is mixed in the right side of the heart (resulting in the formation of mixed venous blood) and pumped into the lungs. In the lungs the blood vessels break up into a net of small vessels surrounding tiny lung sacs (alveoli). The vessels surrounding the alveoli provide a large surface area for the exchange of gases by diffusion along their concentration gradients. A concentration gradient exists between the partial pressure of CO2 (PCO2) in the mixed venous blood (PvCO2) and that in the alveolar PCO2. The CO2 diffuses into the alveoli from the mixed venous blood from the beginning of inspiration until an equilibrium is reached between the PvCO2 and the alveolar PCO2 at some time during breath. When the subject exhales, the first gas that is exhaled comes from the trachea and major bronchi which do not allow gas exchange and therefore will have a gas composition similar to that of inhaled gas. The gas at the end of exhalation is considered to have come from the alveoli and reflects the equilibrium CO2 concentration between the capillaries and the alveoli. The PCO2 in this gas is the end-tidal PCO2 (PETCO2).
When blood passes the alveoli and is pumped to the left side of the heart to the arteries in the rest of the body, it is known as the arterial PCO2 (PaCO2). The arterial blood has a PCO2 equal to the PCO2 at equilibrium between the capillaries and alveoli. With each breath some CO2 is eliminated from the lung and fresh air containing little or no CO2 (CO2 concentration is assumed to be 0%) is inhaled and dilutes the residual alveolar PCO2, establishing a new gradient for CO2 to diffuse out of the mixed venous blood into the alveoli. The flow of fresh gas in and out of the lungs each minute, or minute ventilation (V), expressed in L/min, is that required to eliminate the CO2 brought to the lungs and maintain an equilibrium PCO2 and (PaCO2) of approximately 40 mmHg (in normal humans). When one produces more CO2 (for example, as a result of fever or exercise), more CO2 is produced and carried to the lungs. When CO2 production is normal, the PaCO2 falls if one increases the ventilation (hyperventilation). On the contrary, when CO2 production remains normal, the PaCO2 rises if one increases the ventilation (hypoventilation).
It is important to note that not all V contributes to elimination of CO2. Some V goes air passage (trachea and major bronchi) and alveoli with little blood perfusing them, and thus contributes minimally to eliminating CO2. This V is termed xe2x80x9cdead spacexe2x80x9d ventilation and gas in the lung that has not participated in gas exchange with the blood is called xe2x80x9cdead spacexe2x80x9d gas. That portion of V that goes to well-perfused alveoli and participates in gas exchange is called the alveolar ventilation (VA) and exhaled gas that has participated in gas exchange in the alveoli is termed xe2x80x9calveolar gasxe2x80x9d.
A method of accelerating the resuscitation of a patient has been disclosed in PCT application No. WO98/41266 filed by Joe Fisher. When the patient breathes at a rate such that his ventilation is less than or equal to the fresh gas flowing into the fresh glowing into the circuit, all of the inhaled gas is made up of fresh gas. When the patient""s minute ventilation exceeds the fresh gas flow, the inhaled gas is made up of all of the fresh gas and the additional gas is provided by xe2x80x9creserve gasxe2x80x9d consisting of fresh gas plus CO2 such that the concentration of CO2 in the reserve gas of about 6% has a PCO2 equal to the PCO2 in the mixed venous blood.
The limitation of the above method is that a source of reserve gas and its delivery apparatus must be supplied to pursue the method. The reserve gas must be at about 6% of CO2 concentration substantially having a PCO2 equal to that of mixed venous blood or about 46 mmHg. The portability is thus limited since a sufficiently long tubular structure, typically about 3 m, is required to prevent the atmospheric air from diffusing in and diluting the expired CO2 concentrations. While climbing at high altitude, it would be very difficult to carry oxygen tanks and a long tubular expired gas reservoir. Another example of such a difficulty would be when preventing hyperventilation while ventilating with air in the course of resuscitating newborns and adults in out-of-hospital setting.
The invention provides a isocapnia circuit that maintains a constant PCO2. A flexible container is used to replace the fresh gas reservoir bag used in the prior art. The flexible container is actively collapsed by the inspiratory effort of the patient during inspiration and passively expands during expiration, the atmospheric air is thus drawn into the flexible container and the circuit through a port. The expiratory reservoir is provided with a flexible bag so that the volume of expired gas rebreathed is displaced by collapse of the bag rather than entrainment of atmospheric air, thus preventing the dilution of CO2 in the expired gas reservoir.
Therefore, the benefits of controlling the PCO2 at a constant level are reaped and the expense and inconvenience of supplying fresh gas are not incurred. The compact nature of the isocapnia circuit makes its use practical outdoors, during physical activity and in remote environments. It has been determined that people living at high altitude such as mountaineers, miners, astronomical observatory personnel would benefit from preventing the PCO2 level falling excessively as a result of the involuntary tendency to hyperventilate while they are at high altitude. It would also been determined that resuscitation of newborns with air has demonstrable advantages over resuscitation with oxygen if excessive decrease in PCO2 can be prevented. This therefore was not contemplated in the prior art.
In the invention, the PCO2 is controlled at a predetermined desired level without the need of gas from another source flowing into the circuit under pressure. The expired gas is stored to prevent dilution with atmospheric air such that alveolar portion of the expired gas is rebreathed in preference to dead space gas. The improved breathing circuit can be used to assist patients who are or run the risk of suffering the effects of high altitudes sickness, or who have suffered a cardiac arrest, or who have suffered from an interruption of blood flow to an organ or region of an organ and are at risk of suffering oxidative injury on restoration of blood perfusion as would occur with a stroke or heart attack or resuscitation of the newborn.
In the method of providing a constant PCO2, atmospheric air is aspirated from an inspiratory side to a patient when the patient inhales through the inspiratory side. A gas exhaled by the patient is accumulated in an expiratory gas reservoir connected to an expiratory side, through which the patient exhale. The gas exhaled by the patient and stored in the expiratory gas reservoir is allowed flowing into the inspiratory side to mix with the aspirated atmospheric air when a minute ventilation of the patient exceeds the atmospheric air aspirated to the inspiratory side.
The above isocapnia circuit comprises a breathing port, through which a subject inhales and exhales; an inspiratory port, communicating to the breathing port with an inspiratory valve that allows air flowing to the breathing port and prevents air flowing from the breathing port to the inspiratory port, the inspiratory port having an atmospheric air aspirator to aspirate the atmospheric air therein; an expiratory port, communicating to the breathing port with an expiratory valve that allows air flowing from the breathing port to the expiratory port and prevents air flowing to the breathing port, the expiratory port having an expiratory gas reservoir to store a gas exhaled by the subject flowing across the expiratory valve; and a bypass conduit, communicating the inspiratory and expiratory ports with a bypass valve, the bypass valve allows a one-way flow of air from the expiratory port to the inspiratory port with a pressure differential applied thereto.